CIE1931

CIE 1931 refers to the standard color space and associated color-matching data defined by the International Commission on Illumination (CIE). It is based on color-matching experiments conducted by Wright and Guild in the 1920s and was adopted by CIE in 1931 as a framework for describing colors in a device-independent way.

Two standard observers were defined: a 2-degree field and, later, a 10-degree field to model different viewing conditions. The foundational data are the color-matching functions x̄(λ), ȳ(λ), z̄(λ) measured across the visible spectrum (roughly 380–780 nm). The ȳ(λ) function is closely related to perceived brightness, i.e., luminance.

From these functions, the CIE XYZ tristimulus values X, Y, Z are obtained by linear integration with a given spectral power distribution. The transformation is chosen so that the Y value corresponds to luminance and so that X, Y, Z are nonnegative for practical spectra. The XYZ space is considered device-independent and serves as the standard reference space for color reproduction and conversion.

Chromaticity coordinates are defined as x = X/(X+Y+Z), y = Y/(X+Y+Z), and z = Z/(X+Y+Z). The x,y chromaticity diagram (a projection of the XYZ space) is widely used to visualize the relationship between colors independent of brightness.

Applications and legacy include underpinning modern color management, calibration, and the derivation of perceptual spaces such as CIE Lab and CIE Luv. The model has also informed many device-to-device color conversions (e.g., to sRGB or CMYK). Limitations of the 1931 model led to later refinements, such as the CIE 1964 10-degree observer and uniform chromaticity spaces, but CIE 1931 remains foundational in color science.